OLEDs based on organic polymer materials, also known as polymer light emitting diodes (PLEDs), have drawn much attention due to their potential application in next generation panel displays. Though huge improvement has been achieved in the last years, the performance of the PLEDs, especially the lifetime (particularly for blue PLEDs) still needs further improvement to be commercially successful. Single layer PLEDs, where the hole transport, electron transport and emissive layer are combined into one layer, have the advantage of simple processing, however, they do often show poor lifetime. WO 2004/084260 A2 discloses a PLED wherein the combination of a specific cathode metal with an interlayer between the hole injection layer (HIL) and the light emitting polymer (LEP) is reported to improve the lifetime compared to a conventional single layer PLED.
An “interlayer” as referred to hereinafter means a layer in an OLED device that is situated either between the hole injection layer (HIL) and the emissive layer (EL), or between the electron injection layer (EIL) and the EL, and is intended to prevent electrons from flowing into the HIL, or holes from flowing into the EIL, respectively. An interlayer for use between the HIL and the EL should usually comprise a material having hole transport and electron blocking property, and an interlayer for use between the EIL and the EL should comprise a material having electron transport and hole blocking property.
However, the additional interlayer is undesired in mass production. Also, since its processing is not easy and well controllable, the reliability of the performance of a PLED comprising such an interlayer is usually not sufficient for mass production.
It is therefore one aim of the present invention to find single layer PLEDs having a lifetime of the interlayer system that is comparable or even better than that of prior art PLEDs. Another aim of the present invention is to provide new materials for use in single layer PLEDs, which have advantageous properties, in particular good processability and high lifetime. Another aim the present invention is to extend the pool of PLED materials available to the expert. Other aims of the present invention are immediately evident to the expert from the following detailed description.
It has been found that these aims can be achieved by providing polymer blends as claimed in the present invention. In particular, it was surprisingly found that by using a blend of a hole conducting polymer (hereinafter also referred to as “polymer 1”) and an electron conducting polymer (hereinafter also referred to as “polymer 2”) as LEP in a single layer PLED device, it is possible to achieve a longer lifetime than in a PLED where polymer 1 has the function of an interlayer and polymer 2 has the function of an emissive layer.
WO 2005/053052 A1 discloses a polymer blend comprising a first and a second polymer with triarylamine units, wherein both polymers have hole transport properties. In contrast, the polymer blends according to the present invention are characterized in that the two polymers do transport different types of charge carriers, and preferably either polymer transports only one type of charge carrier.
Morteani et al., Adv. Mater. 2003, 15(20), 1708 and WO 02/28983 A1 disclose a polymer blend comprising a polymer with fluorene units and benzothiadiazole units and a polymer with fluorene units and triarylamine units, and its use in PLEDs. However, the polymers described in these references are suggested to have a bandgap offset smaller than the exciton binding energy, so that the exciton can be stabilised at the blend interface, thus realising a barrier-free heterojunction, and thus a high efficient green PLED using these blends. In contrast, a blend consisting of polymers having a large bandgap offset is usually considered to be advantageous only for use in photovoltaic cells, but disadvantageous for use in PLEDs. This is reported for example by J. J. M. Halls et al., Phys. Rev. B. 1999, 60, pp 5721. Also, WO 02/28983 A1 discloses only a narrow, defined range for the molecular weight of the polymers to be used in the blend.
Birgerson et al., Adv. Mater. 1996, Vol 8, pp 982, “Efficient blue-light emitting devices from conjugated polymer blends”, discloses a device wherein the emissive layer consists of a blend of PDHPT (poly(2,5-diheptyl-1,4-phenylene-alt-2,5-thienylene)) and PDPP (poly(2,5-diheptyl-2′,5′-dipentoxybiphenylene)). U.S. Pat. No. 5,378,519 discloses a PLED containing a compound having a skeleton of triarylamin and having a carbonyl group. The compound can be blended with other polymers as emissive layer. Cimrova et al., in Adv. Mater. 1998, Vol 10, pp 676, “blue light emitting devices based on novel polymer blends” disclose a device wherein the emitting layer consists of PPBSi (poly(phenylbiphenylylsilylene)), P3V (poly(p-terphenyldiyl-vinylene)) and PBD (2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole)). The two polymers have similar HOMO levels. Berggren et al, Nature 1994, Vol. 372, pp 444, discloses PLEDs comprising a polymer blend, wherein the color is varying as a function of the operation voltage. The polymers used in this device are four different thiophene homopolymers.
Cina et al., Proceedings of SPIE Vol. 4279 pp 221, discloses a PLED based on a blend of two polyfluorenes prepared by Suzuki cross-coupling, with the first component being an electron transporter and emitter and the second component being a hole transporter. No information on the polymer composition is given. However, it is indicated in the document that emitter and electron transport unit are given by the same unit in the first component. Morgado, et al., describe in Appl. Phys. Lett. 2002, Vol 80, pp 2436 a PLED using a blend of homo polymer poly(9,9′-dioctyl-fluorene) (PFO) with an alternating polymer poly(9,9′-dioctylfluorene-alt-benzothiadiazole) (F8BT) as emissive layer, and PPV prepared via standard precursor route as interlayer. The same blend is discussed by Wilkinson et al., in Appl. Phys. Lett. 2001, Vol 79 pp 171 for the dependence of performance on pixel dimension. Niu et al., describe a red PLED consisting of a blend of MEHPPV with terpolymers of dioctyl-fluorene (F8), benzothiadiazole (BT) and dithienylbenzothiadiazole (DBT). The MEHPPV is used as the hole transporting component, polymer backbone of the terpolymers as electron transport unit, and BT and DBT as emitter unit. Here all the HOMOs of electron transporting and emitter units are lower than the hole transporting unit MEHPPV. Suh, et al. report in Adv. Mater. 2003, Vol 15 pp 1254, that a blue bispirofluorene polymer can be enhanced by blending it with hole transporting small molecules (HTSM), such as 1,3,5-tris(N,N-bis(4-methoxyphenyl)aminophenyl)benzene (TDAPB), 4,4′,4″-tris(N-3-methyl-phenyl-N-phenylamino)tri-phenylamine (MTDATA), N,N′-di(4-(N,N′-diphenyl-amino)phenyl)-N,N′-diphenylbenzidine (DNTPD) and 1,1-bis(4-bis(4-methylphenyl)amino-phenyl)cyclohexane (TAPC). Here, all of the HTMs have HOMOs similar to the blue polymer. Yong Cao et al., study in Science 1999, Vol. 397 pp 414 the singlet/triplet ratio in a conjugated polymer attained by blending electron transport materials with conjugated polymers, OC1C10-PPV and MEHPPV.
In WO 99/48160 A1, an OLED using a mixture of a hole transporting component (first component), an electron transporting component (second component) and an emissive component (third component) is disclosed, wherein at least one of the first, second and third components forms a type II semiconductor interface, with another of the first, second and third component. The type II interface is defined as an interface in which the minimum energy difference between the highest HOMO and the lowest LUMO state is between levels on different sides of the heterojunction. It is also disclosed that the third component and one of first and second component can be provided as functional moieties, say as pendant group, of the same molecule, say copolymer. However, no further enabling technical disclosure is provided in this direction. In fact, no such blend system with long lifetime has been reported so far.
It was now surprisingly found that by using a blend of two polymers with a large HOMO level offset as claimed in the present invention, a PLED device with improved performance could be achieved, in particular a single layer PLED with comparable or even better lifetime than interlayer PLEDs, thus leading to simple processing and higher reliability in mass production. Also, it was surprisingly found that in the blends according to the present invention even polymers with high molecular weight can be used, which could not be expected in view of the prior art.
The polymer blends of the present invention can be advantageously used both in PLEDs with and without an interlayer. On the one hand, if the polymer blends according to the present invention are used in a PLED device, it is possible to increase the lifetime of the device without the need of an interlayer, so that the interlayer can be omitted and the device assembly can be simplified. On the other hand, if the polymer blends according to the present invention are used in a PLED device comprising an interlayer, they do still significantly improve the performance of said device.